Inductor current emulation circuit for switching power supply

ABSTRACT

An inductor current emulation circuit for a switched-mode power supply (SMPS) which is arranged such that its inductor current (I L ) goes to zero at least once per switching cycle. The emulation circuit includes an RC integrator connected in parallel across the inductor, and a zero reset switch (ZRS) connected in parallel across the integrator&#39;s capacitor. A control circuit operates the ZRS such that it is opened when I L  is non-zero, and is closed for a least a portion of the time during each switching cycle when I L  is zero such that the capacitor is substantially discharged. In this way, the ZRS essentially recalibrates the emulation circuit when I L  is zero. When so arranged, the voltage (V C ) across the capacitor emulates I L . The invention may be implemented with either a discontinuous-inductor-current SMPS, or a continuous-bipolar-inductor-current SMPS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of switched-mode power supplies(SMPS), and particularly to methods of determining the current flowingin the inductor of an SMPS.

2. Description of the Related Art

Switched-mode power supplies (SMPS), which switch voltage to and from aninductor to effect current through it (e.g., to provide a regulatedvoltage output), often use information about the current (I_(L)) flowingin the inductor to control the switching. One way of sensing I_(L) is byadding a current-sensing resistor in series with the inductor; thevoltage across the resistor varies with I_(L). However, the use of asuitable current-sensing resistor adds cost to the SMPS, and power isdissipated as heat in the resistor.

Minimizing the size of the current-sensing resistor lessens theseproblems: a lower resistance reduces power dissipation, allowing the useof a physically smaller resistor which lowers cost. This approach alsohas drawbacks, however. With a lower resistance, a smaller voltage isdeveloped across the resistor, which results in a low signal-to-noiseratio (SNR). Minimizing the resistance value also has the effect ofincreasing the relative impedance of the resistor's unavoidableparasitic inductance. This can cause the voltage across the resistor tobecome distorted with respect to the sensed current. Analog and digitalfiltering techniques have been used to mitigate these problems, butthese increase cost and complexity.

Another technique for producing a signal which varies with I_(L)involves emulating the inductor current using an RC integrator. Aresistor and capacitor are connected in series to form an integrator,which is connected in parallel across the SMPS' inductor. When theresistor and capacitor are properly chosen, the voltage across thecapacitor emulates the inductor current. The accuracy of this approachis optimized over a wide bandwidth when the RC integrator's timeconstant is matched to the time constant of the inductor and itsequivalent series resistance.

Unfortunately, this emulation method has several problems. Theequivalent series resistance of the inductor may not be well-controlledor specified, and can vary substantially over temperature and process.This results in a loss of accuracy. If the time constants are notwell-matched, the emulation circuit can suffer a loss of bandwidth, andthe emulated current signal can become distorted. The mismatch of timeconstants shows up in the emulated current signal as an exponentialdecay of the average of the emulated current signal toward the averageinductor current times the equivalent series resistance of the inductor.

This approach can also result in a poor SNR: the magnitude of theemulated current is given by the inductor current multiplied by theinductor's equivalent series resistance—which is preferably made assmall as possible to minimize power dissipation in the inductor.However, a small equivalent series resistance results in a smallemulated current signal, and thus a poor SNR.

SUMMARY OF THE INVENTION

An inductor current emulation circuit for an SMPS is presented. Theinvention provides accurate inductor current emulation, while looseningthe time constant and equivalent series resistance restrictions found inthe prior art.

The invention is suitable for use with any SMPS which includes aninductor and provides an output current at an output terminal, and whichis arranged such that inductor current I_(L) goes to zero at least onceper switching cycle. The emulation circuit includes an RC integratorconnected in parallel across the inductor, and a “zero reset switch”(ZRS) connected in parallel across the integrator's capacitor. A controlcircuit operates the ZRS such that it is opened when I_(L) isessentially non-zero, and is closed for a least a portion of the timeduring each switching cycle when I_(L) is essentially zero such that thecapacitor is substantially discharged. In this way, the ZRS essentiallyrecalibrates the emulation circuit when I_(L) is zero, therebyeliminating the need to match the time constants of the inductor andintegrator.

When the integrator's components are properly selected, I_(L) is givenby an approximately linear transfer function given by:I _(L) =V _(C) *R*(C/L),where R and C are the resistance and capacitance values of theintegrator's resistor and capacitor, respectively, L is the inductanceof the inductor, and V_(C) is the voltage which develops acrosscapacitor C when the ZRS is open. The invention may be implemented witheither a discontinuous-inductor-current SMPS, or acontinuous-bipolar-inductor-current SMPS, and is applicable to many SMPSconfigurations, including buck, boost, and buck-boost, as well as manyderivative converters.

Further features and advantages of the invention will be apparent tothose skilled in the art from the following detailed description, takentogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic/block diagram illustrating the basic principles ofthe invention.

FIG. 2 is a timing diagram illustrating the operation of the presentinductor current emulation circuit as used with adiscontinuous-inductor-current SMPS.

FIG. 3 is a timing diagram illustrating the operation of the presentinductor current emulation circuit as used with acontinuous-bipolar-inductor-current SMPS.

DETAILED DESCRIPTION OF THE INVENTION

The basic principles of the invention are illustrated in FIG. 1. Atypical SMPS includes a pair of output switches S1 and S2, an outputinductor L, and a filter capacitor C_(o). Inductor L is connected to a“switched-voltage” terminal 5 which is connected to switches S1 and S2,and to a “capacitively-filtered terminal 7 which is connected to filtercapacitor C_(o). A control circuit 10 operates switches S1 and S2 toalternately conduct current to and from inductor L to create a desiredoutput voltage at an output terminal 12; a load 14, represented in FIG.1 with a load resistor R_(L), is driven by the SMPS. The inventionrequires that the SMPS be arranged such that the current I_(L) ininductor L goes to zero at least once per switching cycle under staticoperating conditions, as with, for example, adiscontinuous-inductor-current SMPS, or acontinuous-bipolar-inductor-current SMPS. Note that, though a buck-typeconverter is shown and described herein, the invention is applicable toany SMPS which employs an inductor, including boost and buck-boosttypes, as well as many derivative converters.

An inductor current emulation circuit in accordance with the presentinvention includes an RC integrator 16, and a “zero reset switch” (ZRS)which is controlled by control circuit 10. RC integrator 16 is connectedin parallel across inductor L: a resistor R is connected between L'sswitched-voltage terminal 5 and a node 18, and a capacitor C isconnected between node 18 and L's capacitively-filtered terminal 7. TheZRS is connected across capacitor C such that, when closed, C isdischarged.

Control circuit 10 operates the ZRS such that it is open when I_(L) isessentially non-zero; when the ZRS is open, the voltage across inductorL is integrated by RC integrator 16. Control circuit 10 operates the ZRSsuch that it is closed when I_(L) is essentially zero. The duration ofthe closure need be only a portion of the time when I_(L) is zero, butshould be long enough to ensure that capacitor C is substantiallydischarged. As the invention requires that I_(L) go to zero at leastonce per switching cycle under static operating conditions, the ZRS isclosed and capacitor C discharged at least once per switching cycleunder static operating conditions.

When so arranged, the voltage (V_(C)) which develops across capacitor Cwhen ZRS is open emulates I_(L). V_(C) can then be used by controlcircuit 10 to control the operation of switches S1 and S2 in order toachieve a desired output voltage. When the ZRS closes when I_(L) iszero, it essentially recalibrates the inductor current emulationcircuit. This frees the emulation circuit's time constant from beingmatched to the time constant of inductor L and its equivalent seriesresistance. Thus, the emulation circuit can accurately emulate I_(L)regardless of the time constant of inductor L, or its equivalent seriesresistance.

When the values of R and C are properly chosen, there is anapproximately linear transfer function between capacitor voltage V_(C)and inductor current I_(L), which is given by:I _(L) =V _(C) *R*(C/L)where R, C and L are the resistance, capacitance and inductance valuesof resistor R, capacitor C and inductor L, respectively.

The operation of the invention with an SMPS operated indiscontinuous-inductor-current mode is shown in FIG. 2. Three switchingcycles are illustrated. During each switching cycle, switch S1 is turnedon and conducts current to inductor L such that I_(L) increases. The ZRSis open (i.e., off) during this time, such that V_(C) increases withI_(L).

When I_(L) reaches a predetermined peak level (as indicated by V_(C)),control circuit 10 turns off S1 and turns on switch S2. This causesI_(L) to decrease; because the ZRS is still open, V_(C) decreases withI_(L). When V_(C) decreases to zero—indicating that inductor currentI_(L) of the discontinuous-inductor-current SMPS has reachedzero—control circuit 10 turns S2 off; both S1 and S2 are held off untilcontrol circuit 10 turns on S1 to begin a new switching cycle.

As noted above, the ZRS is turned on (i.e., closed) when I_(L) isessentially zero, for a duration at least long enough to ensure thatcapacitor C is substantially discharged. Once V_(C) is re-zeroed, ZRSneed not remain on throughout the time that I_(L) is zero, but it isgood practice to keep it on until S1 is turned back on.

There are error sources in the inductor current emulation circuit. Forexample, I_(L) may not always be zero when V_(C) is zero, and/or theturn off of S2 may be delayed due to inaccuracies in V_(C). However, byclosing the ZRS every switching cycle as described herein, theaccumulation of errors cycle after cycle is prevented.

Other sources of error between the emulated current via the method ofthe invention and the actual inductor current should be evident to thoseskilled in analog circuit design. For example, an inductor havingnegligible equivalent series resistance would produce a straight lineramp of current, while the emulated current would follow the exponentialdecay of the chosen RC time constant. The difference between theemulated current value and the straight-line approximation is bothsimply calculated and a parameter that can be straightforwardly tradedoff against the emulation signal's magnitude to whatever degree isdesirable or acceptable.

As noted above, the invention is also usable with an SMPS operated incontinuous-bipolar-inductor current mode. Operation in this moderequires the use of a zero-crossing sensor (“ZCS”, not shown), thatdetects when I_(L) crosses zero, and triggers control circuit 10 tooperate the ZRS. Such a sensor would typically be arranged such that itsimply tracks the polarity of I_(L)—i.e., its output is high when I_(L)is high and low when I_(L) is low.

Two approaches to operating an SMPS incontinuous-bipolar-inductor-current mode are shown in FIG. 3; threeswitching cycles are illustrated. For both approaches, during eachswitching cycle, switch S1 is turned on and conducts current to inductorL such that I_(L) increases. When I_(L) reaches a predetermined peaklevel (as indicated by the emulation circuit's output signal V_(C)),control circuit 10 turns off S1 and turns on switch S2. This causesI_(L) (and V_(C)) to pass through zero, thereby switching from onepolarity to the opposite polarity. When I_(L) reaches a predeterminedminimum level (as indicated by V_(C)), control circuit 10 turns off 52and turns on switch S1 to begin a new switching cycle.

Operation of the ZRS is as follows. For the first approach (see theZRS(1), I_(L)(1), and V_(C)(1) traces in FIG. 3), in response to theZCS's detection of a zero crossing, the control circuit turns on the ZRSfor a brief period as I_(L) crosses zero. The control circuit could bearranged such that, in response to the ZCS, the ZRS is briefly closed ona positive-going zero-crossing, on a negative-going zero-crossing, or onboth. For maximum current emulation accuracy, the ZRS would be turned onfor zero time. As this is not possible, the size of the ZRS and thetiming of its closure should be arranged so that the ZRS is turned onfor as short a time as possible—while still substantially discharging Cand recalibrating the emulation circuit, and without introducing asignificant amount of error in the previously-noted transfer functionwhen inductor current information is required by control circuit 10.

For the second approach (see the ZRS(2), I_(L)(2), and V_(C)(2) tracesin FIG. 3), in response to the ZCS's detection of a zero crossing, thecontrol circuit is arranged to turn on the ZRS and discharge C whenI_(L) is of a polarity during which inductor current information is notneeded. For example, the control circuit may not need inductor currentinformation when I_(L) has a negative polarity. In this case(illustrated in FIG. 3), the control circuit is arranged to operate theZRS such that it is turned on and C discharged when the ZCS detectsI_(L)'s positive-to-negative zero crossing, and turned off at I_(L)'snegative-to-positive zero crossing. This would produce a valid currentemulation signal for positive inductor current.

Another technique which can reduce error that might otherwise arise dueto the inability to discharge capacitor C instantaneously is based onthe recognition that, depending on the control technique and/oroperating mode of the SMPS, there may be an acceptably short time periodwhen inductor current information is not needed and whereby it may beadvantageous in anticipation of the inductor crossing zero current toclose the ZRS and then release it again at about the time the inductorwould be expected to cross zero current. This might be accomplished by,for example, adding a switched offset to the ZCS input and a latchconfigured so that the zero crossing would be signaled early via the useof the offset. The latch would extend the apparent instant of the zerocrossing—thereby holding the ZRS closed and providing time for C todischarge; the release of the latch would occur at the actual zerocrossing, i.e., as known via removal of the offset.

Although the ZRS is shown as an idealized mechanical switch, please notethat a practical implementation of the ZRS, such as a FET transistor,will have a non-zero impedance which will limit the rate of discharge ofcapacitor C. The time required to open the ZRS should be kept as low aspossible, since current emulation accuracy is constrained by any delayin opening the ZRS.

A delay in the closure of the ZRS can impose a system limitation, asenough time must be allotted to allow for the closure of the ZRS and thedischarge of C. Note, however, that the ZRS is triggered to close whenV_(C) reaches zero; as such, the time required to discharge C should beminimal.

The design of control circuit 10 as needed to implement the systemtiming shown in FIG. 2 or 3 is straightforward, and suitable designsshould be evident to those familiar with such circuits.

The linear transfer function given above was said to hold when thevalues of R and C are properly chosen. This will be the case when theresulting time constant for the RC integrator is large enough so that,under a condition of maximum peak inductor current, the maximum voltageat integrator node 18 measured with respect to capacitively-filterednode 7 is sufficiently small with respect to the switched voltagesapplied to switched-voltage node 5 so as to avoid unacceptabledistortion or compression of the transfer function. “Compression” occurswhen the ramp rate of the emulated inductor current signal, which isinitially constant, begins to decrease. The magnitude of the compressionof the emulated inductor current signal is zero at a time t=0 whencurrent is first switched to the inductor (S1 turned on), and then isexponentially proportional to the time expired since t=0 divided by theintegrator time constant.

Note that any compression of I_(L) which occurs during the period thatS1 is on is essentially “decompressed” during the period that S2 is on.Thus, regardless of the inaccuracy that may arise due to compression atthe peak inductor current level, choosing a small integrator timeconstant does not affect the accuracy of the emulated inductor currentsignal when I_(L) crosses zero.

For example: 1 μs of current emulation using a 4 μs time constant wouldresult in a 12% lower inductor current emulation signal than the actualinductor current (i.e., 12% compression). Similarly, a 10 μs timeconstant would lower the compression to 5%; each doubling of the timeconstant cuts the compression approximately in half. If 12% compressionis acceptable to a system designer, and if some aspect of the design iseased by using that particular time constant (e.g., a very noisy systemfor which a large emulation signal is especially desirable), then such atime constant choice might be considered to be acceptable. For a systemrequiring more accuracy but not able to afford to compensate for thedistortion, the 12% compression might be unacceptable, and the userwould be obliged to use a longer time constant, e.g. 10 μs, and totolerate the lower signal level.

Since the compression factor is known for a chosen inductor currentemulation time constant, there are ways to compensate for this knowncompression. For example, a current limit threshold could be set to acalculably lower emulation current signal threshold corresponding to theknown compression of the peak current signal. More completely, a currentcontrol system could be designed to compensate for the compressionfactor of the emulation current signal such that, essentially, a linearrelationship is established between the inductor current and the controlsystem. Also, note that it is possible to use amplification of a smallbut accurate current emulation signal (that results from choosing a longtime constant) to provide all the signal magnitude and lowcompression-distortion desired. However, the cost benefit of avoidingcircuitry for compensation or amplification is also a consideration ofthe invention.

The integrator time constant should also be made small enough so thatcapacitor voltage V_(C) is high enough to avoid signal-to-noise ratio oroffset-error-related problems when processed by control circuit 10.

As noted above, the prior art obligates a designer to employ anintegrator time constant that is as large as that of the inductor. Thepresent invention allows the use of a smaller time constant, whichenables the inductor current emulation signal level to be made largerelative to the small offset that might be introduced due to anincompletely discharged C.

Another consideration respecting the integrator's R and C values is theintegrator's impedance. The R and C values should be small enough toenable the emulated inductor current signal to be connected to externalmonitoring circuitry without introducing significant error, and largeenough so as to not unnecessarily waste power in resistor R.

As can be seen from the above discussion, the selection of theintegrator's R and C values is dependent on the requirements of thecontrol circuit 10 which receives the emulated inductor current signal,and on the SMPS' system requirements. The invention enables theintegrator's time constant to be scaled as required for the controlcircuit, while allowing it to deviate from the prior art requirementthat the time constant be matched to that of the output inductor.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. An inductor current emulation circuit for a switched-mode powersupply (SMPS), comprising: a SMPS which includes an inductor havingfirst and second terminals and provides an output current at an outputterminal, said SMPS arranged such that the current (I_(L)) in saidinductor goes to zero at least once per switching cycle under staticoperating conditions, an RC integrator connected in parallel across saidinductor, comprising: a resistor having first and second terminals, saidresistor's first terminal connected to said inductor's first terminal,and a capacitor having first and second terminals, said capacitor'sfirst terminal connected to said resistor's second terminal at a firstnode and its second terminal connected to said inductor's secondterminal, a zero reset switch (ZRS) connected in parallel across saidcapacitor, and a control circuit coupled to said ZRS and arranged suchthat said ZRS is open when I_(L) is essentially non-zero and such thatsaid ZRS is closed during a least a portion of each switching cycle whenI_(L) is essentially zero such that said capacitor is substantiallydischarged, such that the voltage (V_(C)) across said capacitor emulatesI_(L.)
 2. The inductor current emulation circuit of claim 1, whereinsaid RC integrator and said control circuit are arranged such that I_(L)is given by:I _(L) =V _(c) *R*(C/L), where R is the resistor of said resistor, C isthe capacitance of said capacitor and L is the inductance of saidinductor.
 3. The inductor current emulation circuit of claim 1, whereinsaid SMPS is a discontinuous-inductor-current SMPS and said controlcircuit is arranged to receive V_(C) and to close said ZRS andsubstantially discharge said capacitor during a portion of eachswitching cycle when V_(c) is essentially zero.
 4. The inductor currentemulation circuit of claim 3, wherein said SMPS includes first andsecond output switches connected together at a second node, saidinductor's first terminal and said resistor's first terminal connectedto said second node, said SMPS arranged to operate said first and secondoutput switches to effect said switching cycles such that each cycleconsists of: closing said first output switch and opening said secondoutput switch such that current is conducted to said inductor untilV_(C) reaches a predetermined peak value, opening said first outputswitch and closing said second output switch such that current isconducted from said inductor, and opening both first and second outputswitches when V_(C) reaches zero.
 5. The inductor current emulationcircuit of claim 1, wherein said SMPS is acontinuous-bipolar-inductor-current SMPS, said emulation circuit furthercomprising a sensor which detects when current I_(L) crosses zero, saidcontrol circuit arranged to close said ZRS and substantially dischargesaid capacitor when said sensor detects a zero-crossing.
 6. The inductorcurrent emulation circuit of claim 5, wherein said SMPS includes firstand second output switches connected together at a second node, saidinductor's first terminal and said resistor's first terminal connectedto said second node, said SMPS arranged to operate said first and secondoutput switches to effect said switching cycles such that each cycleconsists of: closing said first output switch and opening said secondoutput switch such that current is conducted to said inductor untilV_(C) reaches a predetermined peak value, and opening said first outputswitch and closing said second output switch such that current isconducted from said inductor until V_(C) reaches a predetermined minimumvalue.
 7. The inductor current emulation circuit of claim 5, whereinsaid control circuit is arranged to close said ZRS and substantiallydischarge said capacitor when said sensor detects a negative-goingzero-crossing.
 8. The inductor current emulation circuit of claim 5,wherein said control circuit is arranged to close said ZRS andsubstantially discharge said capacitor when said sensor detects apositive-going zero-crossing.
 9. The inductor current emulation circuitof claim 5, wherein said control circuit is arranged to close said ZRSand substantially discharge said capacitor when said sensor detects anegative-going zero-crossing, and when said sensor detects apositive-going zero-crossing.
 10. The inductor current emulation circuitof claim 1, wherein said SMPS is a continuous-bipolar-inductor-currentSMPS which requires information about I_(L) when I_(L) is of a firstpolarity but not the opposite, second polarity, said emulation circuitfurther comprising a sensor which detects when current I_(L) crosseszero and becomes said second polarity, said sensor arranged to triggersaid control circuit to close said ZRS and substantially discharge saidcapacitor when I_(L) is of said second polarity.
 11. The inductorcurrent emulation circuit of claim 1, wherein said ZRS is a transistor.12. The inductor current emulation circuit of claim 1, wherein said SMPSis configured as a buck-type converter.
 13. The inductor currentemulation circuit of claim 1, wherein said SMPS is configured as aboost-type converter.
 14. The inductor current emulation circuit ofclaim 1, wherein said SMPS is configured as a buck-boost type converter.